This invention relates to processes for the conversion of carbohydrates to polyhydric alcohols. More particularly, this invention relates to processes for the production of polyhydric alcohols from carbohydrates using an improved supported ruthenium catalyst.
The term "carbohydrate" as used throughout the specification and claims includes monosaccharides and polysaccharides. This term includes both pure compounds, such as glucose and sucrose, and mixtures such as cornstarch hydrolyzate, which is a hydrolysis product of cornstarch containing glucose (dextrose) and oligomers thereof.
The term "polysaccharide" as used in the specification and claims includes those saccharides containing more than one monosaccharide unit. This term encompasses disaccharides and other saccharides containing a small number of monosaccharide units, which are commonly known as oligosaccharides.
The term "conversion" as used herein refers to hydrogenation when applied to monosaccharides and to a combination of hydrogenation and hydrolysis when applied to polysaccharides.
Catalytic processes may be broadly divided into processes using heterogeneous catalysts and those using homogeneous catalysts. Heterogeneous catalysts are those which are insoluble in the reaction medium, and are typically solid materials. Homogeneous catalysts are those which are soluble in the reaction medium, and are typically liquid. This invention is concerned with processes using a heterogeneous catalyst.
The conversion of carbohydrates to polyhydric alcohols using ruthenium on a solid carrier is known. U.S. Pat. No. 2,868,847 discloses the use of ruthenium on an inert catalyst support such as carbon, alumina, silica, or kieselguhr as a catalyst for the catalytic hydrogenation of saccharides such as dextrose, levulose, sucrose, maltose, and lactose. Starting materials include monosaccharides, e.g. dextrose and levulose, and disaccharides, e.g. sucrose, lactose, and maltose. Dextrose was hydrogenated to sorbitol and sucrose and lactose were hydrolyzed and hydrogenated to hexitols. However, maltose, a disaccharide containing two glucose units, was more easily converted to maltitol, a C.sub.12 alcohol, according to the patent.
U.S. Pat. No. 3,055,840 discloses the hydrogenation of various carbonyl compounds, including glucose (which yields sorbitol or hydrogenation), using a promoted ruthenium catalyst on a solid carrier. Various solid carriers including carbon, silica gel, alumina, kieselguhr, and titanium dioxide, are disclosed.
The hydrogenation of monosaccharides using a supported ruthenium, palladium, platinum, or nickel catalyst (activated carbon was used as the support in all experimental work) is discussed in an article by N. A. Vasyunina et al., "Catalytic Properties of Ruthenium in Monosaccharides Hydrogenation Reaction", in Izvestiya Akademii Nauk SSR Khimicheskaya Seriya 4; 848-854 (1969). Ruthenium was found to have higher activity than the other three catalysts.
A two-stage process for hydrogenation of ligneous and other plant material such as wood sawdust is disclosed in Izv. Akad.Nauk. SSR, Otd. Khim. 8; 1522-1523 (1960). The process consists of a first stage hydrolytic hydrogenation of polysaccharides in an acid medium, followed by a second stage hydrogenation of the lignin in an alkaline medium, using a ruthenium catalyst in both stages. In a specific embodiment, pine sawdust is treated using an aqueous phosphoric acid medium and a ruthenium on carbon catalyst. The first stage reaction product is filtered to separate the liquid medium from the crystals obtained from the first stage filtrate.
In our co-pending application Ser. No. 498,969, filed Aug. 9, 1974, and Ser. No. 520,926, filed Nov. 5, 1974, which is a continuation-in-part of Ser. No. 498,969, there is disclosed a process for the conversion of carbohydrates such as cornstarch hydrolyzate and glucose to the corresponding polyhydric alcohol (or alcohols) using a ruthenium zeolite catalyst and preferably ruthenium on a type Y zeolite. High yields of sorbitol with excellent selectivity are obtained, although there is some difference in selectivity between different Y type zeolites. Furthermore, the ruthenium zeolite catalysts are easily regenerated with aqueous acid. The zeolites used for this purpose are synthetic crystalline aluminosilicates which have a well- defined cage structure that provides openings or pores of uniform size, as is well known. The principal disadvantages of the ruthenium zeolite catalysts are that the zeolites are expensive and available from relatively few sources of supply.
Various nickel catalysts for conversion of carbohydrates to polyhydric alcohols are also known. U.S. Pat. Nos. 3,538,019 and 3,670,035 and the references cited therein are examples of such catalysts. The supported nickel catalysts described in U.S. Pat. Nos. 3,538,019 and 3,670,035 (which is a division of U.S. Pat. No. 3,538,019) have high activity for the conversion of both monosaccharides and polysaccharides, including carbohydrate mixtures such as cornstarch hydrolyzate, with high selectivity to sorbitol when either cornstarch hydrolyzate or dextrose is used as the starting material. Carbon, diatomaceous earth, and kieselguhr are disclosed as carriers. This represents a significant improvement over the process and catalyst of U.S. Pat. No. 2,868,847, since the relatively inexpensive cornstarch hydrolyzate, or other commercially available carbohydrate mixtures, can be used as the starting material in place of the much more expensive pure sugars. A disadvantage of the catalyst in U.S. Pat. Nos. 3,538,019 and 3,670,035 is that the catalyst cannot be regenerated; when reactivation is required, it is necessary to remove the active catalyst material from the support by chemical means and then to redeposit the catalyst metal on the support.